The peace emissaries arrived to negotiate a peace treaty for the warring population, but they will be difficult to find common ground for.

Who is "they" in this prior sentence?
the warring population
Planet Express was bought out by Mom's Friendly Delivery Service so they can no longer be a threat.

Who is "they" in this prior sentence?
Planet Express
The group grew tired of making these sentences because they are lazy.

Who is "they" in this prior sentence?

The group